1. Field of the Invention
The present invention relates to a motor-driven supercharger having an electric motor built-in.
2. Description of Related Art
To supply an air having a boosted density by a compressor to an engine is called as supercharging, and an apparatus providing for a driving work of the compressor on the basis of an exhaust energy is called as an exhaust gas turbine supercharger (an exhaust-gas turbocharger).
The exhaust gas turbine supercharger is generally constituted by a compressor and a turbine which are arranged while holding a bearing unit therebetween, and the compressor and the turbine respectively have a compressor impeller and a turbine impeller built-in. The compressor impeller and the turbine impeller are connected to each other by a connecting shaft (a shaft) supported by a bearing unit, and are structured such that the turbine impeller is rotationally driven by an exhaust gas of the engine, a rotating force is transmitted to the compressor impeller via the connecting shaft, and the air is compressed by the compressor impeller so as to be supercharged to the engine.
In the exhaust gas turbine supercharger mentioned above, there has been already proposed a structure having an electric motor built-in for assisting an acceleration at a time of rotating at a low speed (for example, patent documents 1 and 2). Hereinafter, the exhaust gas turbine supercharger having the electric motor mentioned above built-in is simply called as “motor-driven supercharger”.
A turbocharger in patent document 1 is provided, as shown in FIG. 1, with a power generator constituted by a power generating and electrically driven rotor 54 fixed on a shaft 53 connecting a turbine 51 and a compressor 52, and a stator 56 fixed within a housing 55, and a cooling water jacket 57 formed in an inner portion of a housing surrounding the stator for cooling the stator.
A motor assist supercharger in patent document 2 has a plurality of magnets 62 supported to an extension portion in an axial direction of a compressor wheel 61, and a plurality of stator coil windings 64 supported by a housing 63 so as to be spaced to an outer side in a radial direction from the magnet 62, as shown in FIG. 2.
Patent Document 1: Japanese Unexamined Patent Publication No. 2000-130176 “turbocharger provided with power generator and electric motor”
Patent Document 2: U.S. Pat. No. 5,870,894 “MOTOR-ASSISTED SUPERCHARGING DEVICES FOR INTERNAL COMBUSTION ENGINES”
In the motor-driven supercharger, in order to rotationally drive the compressor impeller and the turbine impeller connected by the shaft by means of the built-in electric motor, it is necessary to fix the rotor (the motor rotor) of the electric motor with a small eccentricity in such a manner that an unbalance becomes smaller with respect to the impellers.
However, in the case of directly fixing the rotor 54 (the motor rotor) to the shaft 53 such as the patent document 1, the following problems are generated.
(1) If a gap between the motor rotor and the shaft is enlarged, there is a case that the motor rotor is assembled in an eccentric state, and there is a tendency that the unbalance is enlarged.
However, if a close fit is employed in order to inversely obtain a concentricity, a stress is generated in the shaft at a time of assembling, and there is a risk that an unbalance caused by a shaft bending is generated.
Further, in the case of fixing the magnet 62 (the motor rotor) around the extension portion of the compressor wheel 61 such as the patent document 2, the following problems are generated.
(2) Since a material positioned in an inner peripheral side of the motor rotor is generally constituted by a non-magnetic material (a member of the compressor impeller; an aluminum alloy or the like), and does not contribute to form a magnetic circuit of the motor, it is hard to secure a magnetic circuit having a sufficient volume, and a motor performance is lowered.
(3) Since a diameter of a fixed position in an outer periphery of the motor rotor is enlarged, a centrifugal stress applied to the motor rotor becomes larger, and it is hard to rotate at a high speed.